1. Field of the Invention
The present invention relates to a particle sensor for detecting the solid particles contained in a fluid.
2. Description of Related Art
When solid particles are present in a fluid (a liquid or a gas), it is necessary in some cases to detect the presence of the solid particles. Detection of such solid particles is particularly important when the solid particles present in the fluid badly affect the intended action of the fluid.
Lubricating oils (e.g. engine oils) are used in internal combustion engines of automobiles or heavy machinery in order to reduce the frictional resistance and abrasion of the rotating surfaces and sliding surfaces of the engines. Operation of such internal combustion engines generates solid particles (e.g. metal fine particles) caused by abrasion, and the solid particles are taken into the lubricating oils and accelerate, in some cases, the abrasion of the rotating surfaces and sliding surfaces. The solid particles, etc. present in the lubricating oils are ordinarily removed by the use of a filter such as oil filter or the like. The condition of the lubricating oils can be monitored in detail by detecting the solid particles present in the lubricating oils.
Hydraulic oils and flushing oils are used in power transmission mechanisms (e.g. transmission), oil hydraulic pipe systems (e.g. oil hydraulic servo valve), rolling, pressing, etc. It is important to control the condition of these oils by detecting the solid particles present in the oils and/or examining the viscosities of the oils.
It is also important to detect the particles suspended in the air or examine their concentration in the air in order to monitor the level of air pollution. Such detection includes detection of the particles suspended in the exhaust gas discharged from a plant, a factory or the like.
For such detection of solid particles present in a fluid, a particle sensor comprising a sensor element using a piezoelectric film is disclosed in Japanese Patent Application Laid-open (Kokai) No.7-301594. With this particle sensor, solid particles present in a fluid collide with the detecting section (having a piezoelectric film) of the sensor element or with the vibrating section (mounting the detecting section thereon) of the sensor element; thereby, the vibrating section and the detecting section are vibrated; the piezoelectric film converts the vibration into electric signals; the electric signals are outputted by electrodes holding the piezoelectric film between them.
In detecting solid particles present in a fluid by the use of a particle sensor such as mentioned above, the fluid often contains bubbles besides the solid particles when the fluid is a liquid such as oil or the like. The vibrating section of the particle sensor is vibrated as well by the collision of the bubbles and the signals of this vibration are outputted as well from the detecting section. To accurately know the concentration of solid particles in fluid, it is necessary to distinguish the signals given by the vibration caused by collision of bubbles (the signals are hereinafter referred to as "bubble signals") from the signals given by the vibration caused by collision of solid particles (the signals are hereinafter referred to as "particle signals").
The vibration of the particle sensor caused by the collision of solid particles generally appears as the primary resonance vibration of the sensor element, such as shown by the signal waveform of FIG. 14. Meanwhile, the vibration of the particle sensor caused by collision of bubbles often appears as a long-period vibration having a frequency lower than the primary resonance frequency of the sensor element, such as shown by the signal waveform of FIG. 15.
However, collision of bubbles sometimes allows the vibrating section to cause vibrations containing the primary resonance vibration of the sensor element, such as shown by the signal waveform of FIG. 16 or FIG. 17. Such a vibration containing the primary resonance vibration of the sensor element, caused by collision of bubbles is difficult to distinguish from a vibration caused by collision of solid particles; this may result in taking of bubble signals for particle signals when a conventional sensor is used, and has been one reason for the lower accuracy of conventional sensors.
To distinguish the primary resonance vibration caused by collision of bubbles, from the vibration caused by collision of solid particles, there were proposed methods of distinguishing the two vibrations based on the difference in times from starting to reaching maximum amplitude of the primary resonance vibrations of the two collisions, or based on the presence or absence of the vibration components having frequencies lower than the primary resonance frequency, in the primary resonance vibration caused by collision of bubbles. However, none of these methods was sufficient in distinguishability.